The present invention relates to a magnetoresistive sensor for sensing an external magnetic field, and a magnetoresistive head for reading data from a magnetic recording medium. More particularly, the invention relates to a magnetoresistive sensor and magnetoresistive head employing a giant magnetoresistive sensing element biased by an alternating magnetic field.
A giant magnetoresistive (GMR) sensing element comprises a pair of ferromagnetic layers separated by a nonmagnetic layer. The magnetization of the first ferromagnetic layer is pinned in a fixed orientation. The pinning may be effected by exchange coupling with an adjacent antiferromagnetic layer, or simply by the use of a ferromagnetic material with high coercivity. The magnetization of the second ferromagnetic layer, which generally comprises a magnetically soft material, is not pinned, and can be readily altered by an external magnetic field. The GMR sensing element may have a single pair of ferromagnetic layers, or many pairs of layers stacked one over another.
The electrical resistance of a GMR sensing element is affected by spin scattering of conduction electrons in the first and second ferromagnetic layers. The amount of spin scattering depends on the angle between the magnetization directions of the two layers. When the magnetization of the second ferromagnetic layer is aligned with an external magnetic field, the orientation of the external field can be detected from the electrical resistance of the GMR sensing element. Data can be read from magnetic recording media in this way, the external field in this case being a leakage field from the recorded data bits.
In recent disk drives, read heads employing GMR sensing elements have demonstrated significantly higher sensitivity than heads employing previous sensing technologies. The enhanced sensitivity permits data to be recorded at higher densities. GMR sensing elements have also been used in devices such as metal detectors and angular position sensors, where they combine the advantages of high sensitivity and small size.
The performance of conventional GMR sensors and GMR heads is determined to a large extent by the magnetic properties of the second or unpinned ferromagnetic layer. Desired further improvements in sensitivity thus depend on the development of improved ferromagnetic materials, but this has turned out to be extremely difficult.